1. Field of the Invention
This invention relates to a variable power optical system, and particularly to a variable power optical system for enlarging and projecting, for example, an original image onto a projection surface such as a screen in a fashion in which the magnification thereof can be varied.
2. Description of the Related Art
In recent years, along with the extensive utilization of portable computers such as notebook type personal computers and mobile computers, projection type display apparatuses including forward projection type front projectors have been widely used to enlarge and display images prepared on computers in conference or presentation at offices. Further, along with the permeation of digital versatile disks (DVDs) into the general market and the diversification and higher definition of video information distribution such as digital broadcasting, image appreciation on large screens has also come to be done by the use of projectors in homes.
Heretofore, a refraction optical system comprised of a plurality of lenses has been widely used as a projection optical system for a projector. When an optical system of this kind is used as a projection display apparatus like a front projector, the center of the image field is designed to be at a location deviated from the optical axis of the optical system in order to secure an observer's field of view. This is effected with a panel surface made parallel-decentered from the optical axis within an angle ensured by the optical system, that is, by the use of a part of an effective angle of view, and therefore, to shift the screen greatly from the optical axis, it is necessary to let the optical system have a sufficiently wide angle of view. Further, it is also necessary to suppress various aberrations including chromatic aberration of magnification liable to occur strongly with a higher angle of view. Also, regarding rear projection in which projection from the rear is effected onto a screen, the compactness of an apparatus is demanded and therefore, a very wide angle of view is required even if the image field is not shifted from the optical axis. Again in this case, the correction of chromatic aberration of magnification which greatly governs the quality of an image must be done sufficiently.
In such a background, in order to appreciate a large screen under diversified use environments of limited space, brightness and higher definition and yet, further compactness, a wider angle of view, higher variable power, etc. are required of the projection optical system.
Japanese Patent Application Laid-Open No. H11-109227 and Japanese Patent Application Laid-Open No. 2002-182110 realize a wide angle of view by the use of a coaxial optical system. In Japanese Patent Application Laid-Open No. H11-109227, the ratio between the focal length of a total system and the focal length of a front unit is regulated by a construction comprising a front unit and a rear unit to thereby effect the correction of spherical aberration and coma while maintaining the spacing between the units, a back focal length and telecentricity. Thereby, a wide angle of view equal to or greater than a half angle of view 35° is realized while the spacing between the units for inserting a folding mirror thereinto for the downsizing by the layout of the back focal length and the optical system which enables a dichroic mirror to be inserted is secured.
Japanese Patent Application No. 2002-182110, in a zoom lens of a five-unit construction, appropriately sets a diffraction optical element in a predetermined lens unit, and corrects chromatic aberration of magnification and at the same time, achieves the downsizing of the entire lens system. Thereby, this publication proposes a zoom lens securing a variable power ratio of 1.2 or greater and a great aperture of Fno of the order of 2, and yet having its various aberrations corrected sufficiently, and maintaining a back focal length and telecentricity.
On the other hand, in front projection, in order to effect image projection without hampering the observer's field of view and for the downsizing of real projection, there has been made an invention of a projection optical system capable of obliquely projecting onto a screen. However, so-called trapezoidal distortion is caused by obliquely projecting onto the screen and therefore, there has been made an invention for correcting this trapezoidal distortion. For example, in Japanese Patent Application Laid-Open No. H10-282451, the trapezoidal distortion is corrected by the use of an eccentric aspherical surface.
In a non-coaxial optical system, as a designing method therefor and a calculating method for a paraxial amount such as the focal length thereof are shown in Japanese Patent Application Laid-Open No. H09-5650, and examples of the design thereof are shown in Japanese Patent Application Laid-Open No. H08-292371, Japanese Patent Application Laid-Open No. H08-292372 and Japanese Patent Application Laid-Open No. H09-222561, it has become apparent that by introducing the concept of a reference axis and making a constituent surface into an asymmetrical aspherical surface, it is possible to construct an optical system having its aberrations corrected sufficiently. Such a non-coaxial optical system is called an off-axial optical systems (an optical system defined as an optical system including a curved surface (off-axial curved surface) in which, when considering a reference axis along a ray passing through the center of the image and the center of the pupil, a surface normal at the point of intersection of the constituent surface with the reference axis is not on the reference axis, and at this time, the reference axis assumes a shape that the reference axis is folded down). This off-axial optical system is such that the constituent surface thereof generally is non-coaxial and eclipse does not occur even on the reflecting surface thereof and therefore, makes it easy to construct an optical system using a reflecting surface. Also, it forms an intermediate image in the optical system to thereby constitute an optical system having a wide angle of view and yet being compact. Further, it is an optical system having a front stop and yet, enables a design or development of an optical path to be effected relatively freely and therefore can constitute a compact optical system. By making the most of these, Japanese Patent Application Laid-Open No. 2001-255462 and Japanese Patent Application Laid-Open No. 2000-89227 correct the trapezoidal distortion by the use of a rotationally asymmetrical reflecting surface having a curvature.
In Japanese Patent Application Laid-Open No. H11-109227, it is expected that as a wider angle of view progresses to a half angle of view of 40° or greater in the future, it will become difficult to maintain the optical system compact while correcting various aberrations including chromatic aberration of magnification. Further, in an embodiment, the optical system does not have the focal length changing function and therefore, it is good when it is used at a fixed angle of view as in rear projection, but is unsuitable for use in front projection.
In Japanese Patent Application Laid-Open No. 2002-182110, a diffraction optical element is used to thereby decrease the number of lenses and suppress chromatic aberration of magnification, but the variable power ratio is limited to 1.2, and a fundamental solution for increasing the variable power ratio is not done and therefore, a greatly wider angle of view and a greatly higher variable ratio cannot be expected.
Japanese Patent Application Laid-Open No. H10-282451 corrects trapezoidal distortion by a decentered aspherical surface, but the shift amount of the image plane is limited, and from the point that it is not telecentric relative to a liquid crystal panel, it is unsuitable for a projector. Also, Japanese Patent Application Laid-Open No. 2001-255462 and Japanese Patent Application Laid-Open No. 2000-89227 realize the correction of the trapezoidal distortion of a wide angle of view by the use of a rotationally asymmetrical reflecting surface having a curvature, but these techniques are specialized into a technique for effecting oblique projection, and are not an embodiment having the focal length changing function.
Also, in an image pickup system, in Japanese Patent Application Laid-Open No. H09-222561, use is made of an optical element constituted by a refracting surface coaxial with an integrally shaped optical element having surface mirrors decentered to each other, and the relative position of at least two elements is changed to thereby effect a focal length change. However, when ray tracing from an image pickup element side is done, the coaxial optical system effects substantially one-to-one imaging at a wide angle end between it and a reflecting optical system, and at a telephoto end, the magnification borne by the reflecting optical system becomes higher. Therefore, various aberrations caused in the coaxial optical system having the focal length changing function are enlarged by the reflecting optical system and the various aberrations become uncorrectable. Also, the coaxial optical system once images between it and the reflecting optical system, and enlarges the image by the reflecting optical system and therefore, is unsuitable for the downsizing of the optical system even if the sharing of the magnification of the coaxial optical system is heightened to suppress the enlargement of the aberrations by the reflecting optical system.
In Japanese Patent Application Laid-Open No. 2001-264633, use is made of an optical system provided with an optical element having a plurality of optical acting surfaces including at least one reflecting surface, and in which the disposition direction of adjacent ones of these optical acting surfaces is defined as a lengthwise direction, characterized in that the optical element is inserted into or removed from an optical path in a direction differing from the lengthwise direction to thereby change the paraxial amount of the system. This, in addition to the variable power function of the coaxial optical system on a reduction side, makes a reflecting optical block on an enlargement side interchangeable with one having a different focal length, and widens the range of variable power of the entire optical system. However, the magnification when the image pickup element of the coaxial optical system of the embodiment is regarded as an object plane is −0.59 at the wide angle end, −0.87 at the middle and −1.23 at the telephoto end, and particularly at the wide angle end, the magnification borne by the reflecting optical block is high and therefore, this also is an optical system liable to enlarge aberrations.